The present invention relates to a centrifugal fan that collects airflow taken in from a center axis of a motor by a scroll casing and discharges the airflow in a centrifugal direction. The present invention also relates to a casing of such a centrifugal fan.
Centrifugal fans, which use DC brushless motors especially, are widely used to cool electronic components of OA equipment such as a personal computer and a copying machine because they can not only make the motors compact and light in weight but also control air quantity easily due to easy control of the motor.
FIG. 12 is a front view of a conventional centrifugal fan viewing in the motor axis direction, FIG. 13 is a side view of FIG. 12 viewed from the side of an exhaust port, FIG. 14 is a sectional view of FIG. 12 along XIV—XIV line, and FIG. 15 is a front view of the centrifugal fan in FIG. 12 when one of two-part casings is removed.
The illustrated centrifugal fan 1 has a casing 10, an impeller 20 that is rotatabley mounted in the casing 10, and a motor 30 that rotates the impeller 20. A circular air inlet 11 is formed at the front of the casing 10 and a rectangular exhaust port 12 is formed at the side of the casing 10.
The casing 10 is constituted by combining resin made first and second casings 10a and 10b that are divided by a plane perpendicular to a rotating shaft 31 of the motor 30. The air inlet 11 is formed on the first casing 10a. As shown in FIG. 14, a bell mouth 13 is formed along the inner circumference of the air inlet 11. The bell mouth 13 is formed by bending a tip whose thickness is the same as the other portion of the casing 10 inside. As shown in FIG. 14, a cylindrical bearing box 15 made from metal is fixed to the second casing 10b. The bearing box 15 supports the rotating shaft 31 via bearings 14 in its inside. A stator 32 of the motor 30 is fixed to the outside of the bearing box 15. Further, a cylindrical partition 16 is formed inside the second casing 10b over 360 degrees. The diameter of the partition 16 is almost identical to that of the motor 30 and the height thereof is constant over all circumferences.
The motor 30 is an outer-rotor type DC blushless motor that consists of a stator 32 having a stator core 32a and coils 32b wound in slots of the stator core 32a, and a rotor 33 having a cup-shaped hub 33a fixed on the tip of the rotating shaft 31, a yoke 33b attached to inner circumferential surface of the hub 33a, and a permanent magnet 33c held by the yoke 33b. Further, a circuit board 34 on which a drive circuit to control power distribution to the coils 32b is contained is fixed to the bearing box 15 at the position between the second casing 10b and the stator 32.
The impeller 20 is formed as a single unit with the hub 33a of the rotor 33, and many blades 21 are arranged on an outer circumference of the impeller 20.
As shown in FIG. 15, the inner circumferential surface of the casing 10 is formed like a scroll and the width of an airflow path, which is formed between the inner circumferential surface of the casing 10 and the outer circumference of the impeller 20, in the radial direction gradually increases from a nose 12a of the exhaust port 12 as a starting point in the rotating direction of the impeller 20 shown by the arrow in the drawing.
When the centrifugal fan 1 is used, the impeller 20 rotates in the counterclockwise direction shown by the arrow, which discharges the air taken in from the air inlet 11 to the periphery by the centrifugal force. The air is collected by the inner circumferential surface of the casing 10, and is discharged from the exhaust port 12.
Incidentally, since the upstream side of the airflow path into which air flows is connected to the downstream side thereof from which air discharges in the above centrifugal fans 1, the discharge airflow becomes turbulent flow, which causes noise and loses discharge pressure.
Japanese Unexamined Patent Publication No. 7-091400 discloses a technique to form an auxiliary air inlet, which is connected to the most upstream portion of the airflow path, on the casing in order to prevent the turbulence of the discharge airflow. However, since the formation of the auxiliary air inlet requires a large change of the casing design, it becomes difficult to divert existing parts or the like.
The above-mentioned centrifugal fan 1 forms the partition 16 on the second casing 10b as shown in FIG. 14 to prevent such turbulence of the discharge airflow. When the partition 16 is formed at the position close to the inner edge of the impeller, the high-pressure air that must be discharged from the exhaust port 12 does not leaks to the low-pressure air at the most upstream side of the airflow path, which can prevent the turbulence of the discharge airflow.
However, since the partition 16 is formed so as to surround the circumference of the circuit board 34 as shown in FIG. 14, there arises a problem of disturbing the airflow to the circuit board 34 and disturbing heat radiation of the electronic circuit on the circuit board (the first problem).
On the other hand, Japanese Unexamined Patent Publication No. 7-46811 discloses a technique regarding heat radiation of an electronic circuit. The publication discloses the technique to provide a metal housing cover in addition to a casing and to contact a power element that generates the largest heat in the electronic circuit with the housing cover to radiate heat of the power element to the outside. However, the technique disclosed in the publication requires a new metal part that must be unified with the resin made casing, there is a problem of increasing manufacturing cost (the second problem).
Further, since the bell mouth 13, which forms the circumference of the air inlet 11, is formed so that its cross-sectional shape becomes an arc from the external surface and the tip side of the bell mouth 13 that faces to the blades 21 of the impeller 20 is formed in the shape of a cylinder in the above-mentioned conventional example, a whirlpool S1, (see FIG. 14) occurs near the tip of a blade 21, which causes a problem of generating noise due to pulsation of airflow near the air inlet 11 (the third problem).
Furthermore, since the above-mentioned conventional example cannot radiate heat generated by the coils 32b of the stator 32 to the outside, the heat reaches the bearings 14 through the metal bearing box 15, which causes a problem of shortening the useful life of the bearings 14 (the fourth problem). This is ascribable to the following reasons. That is, since the head of the hub 33a has comparatively wide surface perpendicular to the rotating shaft 31 in the conventional centrifugal fan 1, the air taken into the air inlet 11 from the outside flows from only the periphery of the hub 33a as shows by the arrow S2 in FIG. 14, and there is little airflow along the portion covering the coils 32b. Further, since the hub 33a is formed by resin molding together with the blades 21, it shows low thermal conductivity.
Furthermore, when the end portions of the exhaust port 12 that are connected to the inner circumferential surface of the casing 10 are flat at both the nose side and the anti-nose side as the above-mentioned conventional example, the discharge pressure is reduced in the exhaust port in the case of the short exhaust port especially, which arises a problem of reducing an air velocity and air quantity (the fifth problem).